Birefringent filter for color television



June 1954 A. H. ROSENTHAL 2,680,146

BIREFRINGENT FILTER FOR COLOR TELEVISION Filed May 31, 1950 2Sheets-Sheet 2 Fig.6. 7. i 62 .fldolph JiRoQgenihaZ,

as, MMILE WCMM Patented June 1, 1954 UNITED STATES PATENT OFFICEBIR-EFRINGENT FILTER FOR COLOR TELEVISION New York Continuation ofapplication Serial No. 580,325, March 1, 1945. This application May 31,1950,

Serial No. 165,311

1 Claim.

The present invention relates to the production of images in color.

This case is a continuation of my application, Serial No. 580,325, filedMarch 1, 1945.

An important object of the invention is to produce images in naturalcolor, for example, in connection with television, by the use ofextremely simple and efficient apparatus.

Many systems of color television are based on successive colorseparation. The partial images are successively formed in black andwhite and each partial image is viewed or projected through suitablecolor filters, which successively are red, green and blue.

In some cathode ray tube television systems these filters are mounted ona rotating disk which is arranged in front of the screen of the cathoderay tube and the rotation of the disk is controlled by synchronizingsignals assuring that each black and white partial image on thefluorescent screen of the tube will be presented through the propercolor filter. In direct-view systems with a cathode ray tube of largersize, a rotary filter disk becomes very bulky and its rapid rotation isnoisy and requires considerable power.

The present invention concerns a color filter of large size, the coloror spectral characteristic of which can be quickly changed to assumesuccessively the primary colors desired.

Other objects and advantages of the invention will be apparent from theaccompanying drawing, wherein Figure 1 is a side elevation of anembodiment of the invention.

Figure 2 is a sectional view on the line 2-2 of Figure 1. Figure 3 is adiagrammatic view of a modification and showing a birefringent elementin front elevation.

Figure 4 is a view similar to Figure 2 and showing a furthermodification.

Figure 5 is a side elevation of a further modification.

Figure 6 is a view similar to Figure 2 of another modification, and

Figure 7 is a side elevation of an additional modification.

Referring to Figure 1, the numeral It designates a cathode ray tubeincluding a fluorescent screen I 1. Two sheets of polarizing material incheated by the numerals l2 and I3, respectively, are positioned betweenthe screen I! and the area at the right of Figure 1 from which thescreen is to be viewed, the eye 9 representing iii a viewing station thedirections of polarization of sheets l2 and I3 being crossed. Betweenthe sheets I2 and I3 is supported a birefringent sheet [4 having asuitable thickness as hereinafter described. Suitable media to comprisethis sheet are various plastics, including cellu losic sheets such ascellophane. Such materials usually have lengthy molecular structurewhich can be oriented in preferred directions by tensions during orafter manufacture. In other words, the materials are photoelastic.

The sheet M of Figure 1 is shown as co-extensive with the area of theface of tube Ill and is supported at its upper edge by a rigidly mountedbar It and a second bar 16 is secured to the lower edge of sheet [4. Anarmature I1 is scoured to the bar 16 by a rod l8. As best shown inFigure 2, a lever l9 pivoted at 20 has its free end pivoted to rod IS.The lever I9 is urged upwardly by a spring 2!, with upward movementlimited by a stop 22. An electromagnet 23 is positioned beneath armatureI1 and when magnet 23 is energized, armature l1 and bar It will be drawndownwardly to thereby place sheet 14 under tension, the degree oftension being controlled in any suitable way, for example, by thecurrent applied to magnet 23 through the leads 24 and 25. These leadsmay extend to a means such as 26 for synchronizing the magnet with thecolor field signals also applied to the tube I 0.

With the preferred polarizing directions-of birefringent sheet l4oriented 45 with respect to the directions of polarization of the twopolarizing sheets l2 and [3, which are preferably crossed to each other,application of tension to sheet M will cause the assembly of sheetsi2,i3 and M to become a color filter. In some instances, sheet l4 may havesuch characteristics, for example, due to its molecular arrangement andstress provided during manufacture. that, in conjunction with sheets l2and I3, it is a color filter, for instance, for transmission of red,when not under tension in the apparatus. In such case, application of afixed degree of tension will deform it to cause it to becometransmissive of green, while a different, e. g., stronger tension willmake it transmissive of blue. In other cases, its color transmissionsmay be established by three different tensions, all in the samedirection.

The magnet 23 may be energized to place the sheet l4 under one or moredesired degrees of tension by the current impulses applied to magnet 23.By having these impulses suitably synchronized with the field signalsacting also on cathode ray tube In in producing successive par- 3 tialimages in black and white, the elements l2. l3 and [4 will serve asthree successive color filters, and the image visible from the right ofFigure -1' will: appear in natural colors when viewed in incident whitelight.

Instead of being perpendicular to each other, the directions ofpolarization of the polarizers' l2 and [3 may be parallel. Also, in thiscase sheets l2, l3 and i4, when sheet i4 is placed under a certaindegree of tension, will only be transmissive of a band of light ofsubstantially one color. Other degrees of tension will render themtransmissive of other" colors. By'su'cces sively applying such tensions,natural color images will be produced.

Figure 3 diagrammatically illustrates a rhodi' fication whereby a sheetof birefringent material I la is deformed by being placed under tensionsexerted along lines H and V which are, for ex"- ample, at 90 to eachother. With such an arrahgement, and with the polarizers crossed,- sheetMa i'ncomb'ination with'sheets l2 and l3'may be transmissive of onecolor inits-norm'al state, and made transmissive of two' other colorswhen stretched along'l-l-or V, respectively. Alternatively, thecombination ofsheets I2, 1 3' and I4 may be made transmissive ofdiiferent colors by applying'different tensions to sheet Ma along lineH, for example. In any event, the tension applying means will besuitablysynchronized with the partial image 'producedon the picture screen suchas H of Figure 1 to-pro'duce images in natural color.

Figure-4 shows a modification whereby several degrees of tension may beexerted on a birefringentsheet Mbby means of three magnets 36', 3 1 and32"individually'ac'tin'g upon armatures 3 3, 34' and 35, respectively.The armatures are'secured to a -bar' [3b secured to one-edge of sheet Nband pivoted at Mic. The armatures maybe placedat-such-positions -on-bar'i615 and the magnets 30 31 and 32' may'h'ave such characteristics,forinstance; number of-windings, that the same current successivelyappliedto each one, for instance, by electronic switchingmeanswell-k'nown per se and controlled bythe field synchronizingimpulses, will exert differentdegreesof tension on sheet Mb and therebyproduce images" in natural 00101.

Figure 5 shows a further modification wherein 1- one behind the otherand may be of the same i.

materialbut'ofdifferent thicknesses, or'may be of diirerent degrees ofbirefringence; In any event, each sheet would normally be freetobirefrin gence; It-will be observed that if the three sheets aresuccessively placed under' suitable tensions by synchronized currentimpulses to the respective magnets 50, 5! and'52g the screen imagewill'appearin natural color.

Figure'G shows a modified arrangement for ex er'ting varying tensions ona sheet of birefringent material. In this instance, an elongatedferromagnetic rod or tube 55 is secured to the lower edge of sheet Mg.Member 55-extends into a coil 56 and will be subjected to varying forcesproducing varying tensions in sheet la in proportion to the currentflowing in that coil.

In" connection with all ofthe above embodiments' of the invention, itwillbe noted that \vitl'i -a-birefringent sheet'such-as I l having athickness 'cl and one index" of refraction noand another index ofrefraction m, for the ordinary and extraordinary rays, respectively, andwith the polarizers crossed, only light of wavelengths (with n=1, 3, 5,et cetera) is allowed to pass through sheets I2, l3 and I4. With small nvaluesit canbe arranged that for slightly varying values (neno) thetransmission band shifts through the whole visible spectrum, and threesuitable (usm) values can be chosen so that the'filter is blue,r'ed orgreen, dependent upon that value} If the polarizers are arranged withtheir axes paralleL/n. would-be 0, 2, 4, 6, et cetera.

It willbe-seen' that it is advisable to make the sheet 14 of a materialthe birefringency of which changes markedly with changed tension, andfurther has a large dependency on the optical wavelength; i. e., a largedispersion of hirefringency. Cellophane is an example of a materialhaving these characteristics.

Throughout the foregoing, reference hasbeen made to placing thebirefringent'sheet under a state of strain-,or deforming it bystretching or expansion, 1. e.,-straining' it in the direction of thebirefringent" element. However, in some in stances, as with certain'stiff but elastic materials such asa thin sheet ofglasspthe deformationmay be perpendicular to the sheet. Figure shows a modification of theFigure 1 structure including a sheet Gil o'f -thinglas's positionedbe'-' tween polarizers l2 and [3. With the lower edge of sheet 60 heldfirmly asby the fixed-clamp 6|, sheet 60 can be successively bentordeflected to different degrees in th'e'direc'tion o'f the arrow A bymeans of a small can'i- 62 including three successive rises. Cam 62canber'otated' a't high sp'eed by a small motor, not shown, rotated insynchronism withthe color field signals ap plied to the c'athoderaytube.

Instead'of using a rotary cam, the sheet 60' could be deflected byanelectromagne't or coil-as inFigure 6, acting upon an armature securedto' the free edge of the sheet, and with the arrn'a' ture movable in thedirection ofthe arrow A' -of Figure '7.

It' Willbe understood fromthe foregoing that the present inventionwillbe highly eflicie'nt-in producing colored images and without any largerapidly moving'oper'ating elements. If magnets are used to place thesheets under tension-, 'they may be extremely small and, in any event,can

be operated indefinitely without wear and re--' sultant noise; Inaddition, magnets can'be operated at the relatively high speed necessaryby small current flow controlled'through electronic tubes and:insynchronization with the partial pictures on the cathode raytube-screen";- If a motor-driven rotary camis used, these parts can berelatively small.

Although the invention has been primarily described in connection with atelevision cathode ray tube, it will be obvious that it may be used withother types of television apparatus as well as with motion pictureprojectors or, in fact, any situation where colored images are tobeproduced.

Any long periodchanges in birefringence may be easily compensated byslight alterations of the actuating currents at suitablecontrolaand withsuitable test colors which-may. be-occasionally transmitted and comparedto colorsin a colornormal.

The terminology used in the specification is for the purpose ofdescription, and not of limitation, the scope of the invention beingindicated in the claim.

I claim:

In an apparatus for producing images in color, means including a viewingtube face to successively produce partial images in black and whiterepresentative of successive colors, a pair of light polarizing elementsin front of said means with the respective axis of polarization thereofat right angles to each other, a birefringent element interposed betweensaid polarizing ele ments, the polarizing elements and birefringentelement being coextensive in area with the area 6 the respective forcesin synchronism with the production of partial color images by saidfirstnamed means, means whereby the forces are of such order in therespective directions that the images will be visible in differingcolors.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,010,307 Leishman Aug. 6, 1935 2,011,553 Land Aug. 13, 19352,014,688 Mabboux Sept. 17, 1935 2,070,787 Frooht Feb. 16, 19372,077,031 Birch-Field Apr. 13, 1937 2,109,540 Leishman Mar. 1, 19382,118,160 Cawley May 24, 1938 2,163,530 Thieme June 20, 1939 2,174,269Land Sept. 26, 1939 2,255,933 Land Sept. 16, 1941 2,350,892 Hewson June1944 2,493,200 Land Jan. 3. 1950

